Making Stereo amps into Mono

You have several alternatives. First, you could get more efficient speakers. A 3dB increase in sensitivity is equivalent to a doubling in power amp output. Second, I like your suggestion of getting another amp, but rather than convert to mono, I suggest going with a passive biamp setup. Third, you could find someone willing to rip the amp apart and then put it back together in better sonic shape than we you started. Not impossible, but certainly not probable and possibly more costly than you think. Finally, you could dump the Nakamichi and purchase a newer, higher powered Pass designed product. It will sound better, but it is the most costly alternative. My suggestion is to try #2 and if you're still not satisfied, go to #4. At the same time, give #1 some thought.

Find a pair of G.A.S. products called "The Bridge" (I'm showing my age). This product allowed two channel amps to be bridged into mono. The bad news: Probably hard to find. The good news: If you can find 'em, they should be cheap.

Bad idea to bridge a stereo into mono. The tonal balance of the amp will change and will result in inferior sound. I would suggest you find another PA7 and use them to Bi-amp your speakers. This way, you power the lows and highs saperately....way better than to bridge.

Bridging not properly done can result your amp to be "fried". Old Nelson Pass designs are famous to blow all outputs at once! (very costly repair bill).